1. Field of the Invention
The present invention generally relates to a gas sensor having a laminate or multilayer structure comprising at least two solid electrolyte layers and an alumina substrate, for use in an internal combustion engine, particularly to a full-range air-fuel ratio sensor (or rather a universal exhaust gas oxygen sensor) capable of measuring air-fuel ratio of an internal combustion over the entire range thereof, a nitrogen oxide gas sensor, a flammable gas sensor capable of measuring carbon monoxide or hydrocarbon, a compound gas sensor capable of measuring plural gases selected from oxygen, nitrogen oxides, carbon monoxide, hydrocarbon gas and other gases.
Specifically, the present invention relates to a gas sensor having a co-fired laminate of at least two zirconia solid electrolyte layers for electrochemical cells and an alumina substrate for embedding a heating resistor, for use, for instance, as a full-range air-fuel ratio sensor for controlling an air-fuel ratio from fuel-lean to rich in an internal combustion engine control, etc.
2. Description of the Related Art
Increasingly strict regulations have been imposed on the emission quantity of harmful matter (e.g., hydrocarbon gas, carbon monoxide, and nitrogen oxides) contained in exhaust gas discharged from an internal combustion engine of an automobile, etc. Moreover, in view of the greenhouse effect and other problems, the necessity to reduce the emission of carbon dioxide has arisen, thereby raising an urgent need for a method of further reducing consumption of fuel by internal combustion engines.
Under such circumstances, more strict requirements have been imposed on gas sensors, which are indispensable for decreasing harmful matter contained in exhaust gas and improving fuel efficiency of the internal combustion engines. In particular, in recent years, demand has arisen for a gas sensor that can activate quickly and can save electric power, while providing improved performance and reliability as well as reduction in size and cost.
U.S. Pat. No. 4,765,880 discloses a configuration of a two-cell gas sensor, including an oxygen-pumping cell and an oxygen-detecting cell. This type of two-cell gas sensor enables full-range measurement of air-fuel ratio of an internal combustion engine of an automobile, to thereby improve fuel efficiency of the internal combustion engine.
USPAP 2001/0047937 A1 discloses a multilayered air-fuel ratio sensor including solid electrolyte substrate layers and at least one heterogeneous boundary layer for absorbing thermal shock or stress interposed between the solid electrolyte substrate layers.
EP 1026502A2 discloses a one-cell type gas sensor including an alumina substrate laminated integrally with an oxygen-ion conductive solid electrolyte layer containing alumina. U.S. Pat. No. 4,733,056 discloses a technique for preventing ion-migration in a ceramic heater per se.
3. Problems Solved by the Invention
In the case of a plural cell-type sensor such as a full range air-fuel ratio sensor that requires an oxygen-pumping cell, an oxygen-concentration detecting cell, a heater for heating the cells and a cavity or space into which oxygen is pumped in or out by the pumping cell, various problems relating to activation of the sensor cells by the heater under a limited electric power consumption, an oxygen-pumping capability of the oxygen-pumping cell, measurement accuracy of the oxygen-detecting cell, reliability of the sensor, etc., arises. This is because the structure and function of the plural cell-type sensor are extremely complicated, compared to a single cell-type sensor.
In addition, electrochemical and structural weakness caused by metal-ion migration, oxygen-ion leakage between the cells, reduction or rather deoxidization of a solid electrolyte layer that constitutes the oxygen-pumping cell, etc., will be problematic in this plural-cell type sensor.
Further, when the gas sensor is designed to adopt a laminate (or multilayer structure) comprising plural zirconia oxygen-ion conductive ceramic layers and an alumina ceramic substrate (for a heating resistor to be embedded therein), etc., a serious problem such as cracks induced in the laminated zirconia ceramic layers arises due to thermal expansion difference between the zirconia ceramic layers and the alumina substrate.
A conventional two-cell type sensor used in an actual automobile combustion engine control system has been composed of two portions (namely, zirconia-sensor cells and a heater-embedded alumina substrate) bonded by a comparatively thick glass of about 200 micrometers which absorbs stress caused by the thermal expansion difference therebetween. This means at least two firing processes (one for the zirconia cells and the other for the heater-embedded alumina substrate) are necessary, resulting in a costly sensor with a slow activation of the sensor cells due to use of glass having lower thermal conductivity, as compared to alumina. Another conventional two-cell type sensor has been composed of zirconia-sensor cells and a heater-embedded zirconia substrate, which also has a disadvantage in the activation of the sensor cells due to use of the heater-embedded zirconia substrate that has insufficiently low thermal conductivity, as compared to the alumina substrate.
From a view point of activation of the sensor cells, the heater has been conventionally attached closer to the oxygen-pumping cell than to the oxygen-detecting cell. In this manner, the temperature of the oxygen-pumping cell can be elevated faster than that of the oxygen-detecting cell. This is because the cavity of the two-cell type sensor delays activation of the sensor, as compared to a single cell type sensor. If electric power for heating the heater is increased for quick activation of the sensor cells, durability and endurance of the heater is sacrificed. If the size of the sensor is made too small, the pumping capability of the oxygen-pumping cell becomes insufficient for accurate determination of the air-fuel ratio of the internal combustion engine. Because of these disadvantages, prior investigators have not been widely successful in providing or incorporating a two-cell type gas sensor into an automobile internal combustion engine and/or an exhaust gas control system thereof.